1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor, an electrophotographic member, a process cartridge and an image forming apparatus that are applicable to an electrophotographic apparatus using an electrophotographic process, such as a copying machine, a printer and a facsimile.
2. Description of the Related Art
In a typical electrophotographic process, after the surface of an electrophotographic photoreceptor formed by utilizing a photo-conductive substance (hereinafter, sometimes referred to simply as the “photoreceptor”) has been evenly charged, a latent image is formed through exposure, and the latent image thus formed is developed by using toner to form a toner image, and after the toner image on the surface of the photoreceptor has been transferred onto the surface of a transferring material such as paper by using an intermediate transferring member or without using this, the transferred image is heated, pressed or heated while being pressed, or subjected to solvent vapor and the like so as to be fixed; thus, a fixed image is formed after having been subjected to these processes. Residual toner on the surface of the photoreceptor is cleaned by cleaning member, if necessary, and again supplied to the above-mentioned processes.
With respect to the surface of the above-mentioned electrophotographic photoreceptor, various characteristics, such as chemical resistance against ozone and NOx generated at the time of charging and a mold-releasing property for improving transferring efficiency, as well as a surface-sliding property, an anti-abrasion property and hardness with respect to mechanical cleaning processes, are required. For this reason, conventionally, a method has been proposed in which a resin layer, formed by dispersing fluorine resin particles such as polytetrafluoroethylene (PTFE), is placed on the surface of a photoreceptor as a surface protective layer (for example, refer to Japanese Patent Application Laid-Open (JP-A) No. 63-56658 and JP-A No. 63-65450). Such a resin layer makes it possible to reduce the frictional coefficient on the surface of the resin layer, to improve the cleaning property, and also to improve the endurance against abrasion. Moreover, it becomes possible to shield a charge-transporting layer and a charge-generating layer that are susceptible to degradation by ozone from outside air, and consequently to improve chemical endurance.
However, since the fluororesin particles used in the fore-mentioned resin layer are not sufficient in the water repellent property thereof, and since the binder resin is exposed on the surface of the resin layer, it is not possible to express a sufficient mold-releasing property. Moreover, a method which polishes the resin layer having the dispersed fluororesin particles so that more portions of the fluororesin particles are exposed has also been proposed; however, the resulting effects are not so great in comparison with its increased costs. Furthermore, in some cases, it is only possible to obtain a contact angle that is equivalent to the application of a single binder resin in the case of the range of the fluororesin content that is practically applicable.
Moreover, in an attempt to increase the fluororesin particles so as to increase the mold-releasing property, fogging is generated on the surface of the photoreceptor to cause degradation in the sensitivity therefore as a result, it causes the degradation in the image quality. In the case of an electrophotographic photoreceptor, the smaller the contact angle, the greater the subsequent surface free energy; consequently, the following problems tend to occur: degradation in the mold-releasing property and the subsequent increase in residual toner from transferring processes (degradation in the transferring efficiency), an increase in surface contamination due to NOx and the like, degradation in the cleaning property due to degradation in the surface sliding property and re-adherence of toner to the surface of the photoreceptor.
Here, the cleaning member as the electrophotographic photoreceptor is used for removing residual toner on the surface of the photoreceptor. The residual toner on the surface of the photoreceptor has a charge, and is allowed to adhere to the surface of the photoreceptor by a strong electrostatic attracting force; therefore, in order to overcome such an electrostatic attracting force to remove the toner from the surface of the photoreceptor, it is necessary to press a blade-shaped cleaning member onto the surface of the photoreceptor with a great pressure.
Since this blade-shaped cleaning member (cleaning blade) requires properties such as superior chemical resistance, anti-abrasion property, moldability and mechanical strength, urethane rubber which is provided with such properties has come to be widely used. However, since a cleaning blade made from urethane rubber has a great coefficient of friction against a photoreceptor whose surface layer is made from a polymer resin such as polycarbonate, a great frictional force is exerted between the photoreceptor and the cleaning blade so that the cleaning blade tends to reversely rotate along the rotation direction of the photoreceptor (blade peeling), resulting in a failure in driving the photoreceptor, as well as making the cleaning process inoperable.
For this reason, fluororesin fine powder or the like having a lubricating property is applied to, or dipped onto an edge portion of the cleaning blade to form a lubricating layer to reduce the coefficient of friction so as to solve the above-mentioned problems. However, this method in which the lubricating layer is formed by using the fluororesin fine powder requires a lubricating layer forming process in addition to a normal cleaning-blade manufacturing process, resulting in complex working processes.
Here, a cleaning blade, which has an edge portion on which a low friction layer mainly composed of rubber and resin is formed, has been proposed (for example, refer to JP-A Nos. 63-27928, 2-101488 and 2-107983). This cleaning blade is formed by applying a low-friction-layer forming material prepared by mixing materials such as silicone powder, fluororesin powder and polymethyl methacrylate (PMMA) powder into a binder such as urethane rubber, silicone rubber, silicone resin, fluororubber, fluororesin and nylon onto a photoreceptor contact portion (edge portion) of the cleaning blade through a dipping method or the like, thereby providing a low friction layer thereon. With this arrangement, it becomes possible to reduce friction between the photoreceptor and the cleaning blade, and consequently to prevent the cleaning blade from being reversed even in a state in which no toner exists on the surface of the photoreceptor, such as a copying process starting state. However, the cleaning blade on which the above-mentioned low-friction layer is formed tends to cause an insufficient reverse-preventing operation in the case where the above-mentioned binder is made from rubber, and also tends to damage the surface of the photoreceptor in the case where the binder is made from resin.
Moreover, many methods have been proposed in which powder or a liquid-state lubricant is added to urethane rubber that is a material forming the cleaning blade so as to improve the lubricating property (for example, refer to JP-A Nos. 1-279282, 3-269564 and 4-93973). However, for example, the cleaning blade which is made of urethane rubber obtained by adding a powder-form lubricant becomes hard, and tends to damage the photoreceptor. Moreover, the application of the cleaning blade made of urethane rubber obtained by adding the liquid-state lubricant tends to cause another problem in which the liquid-state lubricant exudes on the surface of a cleaning blade and contaminates the photoreceptor. As described above, at present, an effective method for preventing the cleaning blade from reversing at the initial state upon starting a copying operation has not been obtained yet. Moreover, in order to improve the endurance of the cleaning blade, a method in which a plasma polymerization film is used has been proposed; however, since the cleaning blade obtained in this method has a plasma polymerization film that is formed on only one surface including an edge portion, it is not possible to prevent the cleaning blade from being reversed.
Moreover, in the case where the intermediate transferring member, which is abutted on the photoreceptor, and allows a toner image on the surface of the photoreceptor to be transferred thereon, has a low mold-releasing property on its surface, “an image loss phenomenon” tends occur during the transferring process. The following items are listed as reasons for the occurrence of this “image loss phenomenon”.                The toner layer becomes thicker in the full-color image forming apparatus.        The application of a roller-shaped transferring member causes a high contact pressure, resulting in a strong mechanical adhesive force between the surface of the intermediate transfer member and the toner.        Since the image-forming processes are executed repeatedly, a so-called filming phenomenon in which toner adheres to the intermediate transferring member in a film shape tends to occur to generate a strong adhesive force between the surface of the intermediate transferring member and the toner.        
With respect to means for avoiding “the image loss phenomenon” caused by the above-mentioned reasons, the following methods have been proposed (for example, refer to JP-A No. 2002-23514).    (1) A roller (conditioning means), which is formed by a member made from a silicone-based material having surface energy that is smaller than the surface energy of the intermediate transferring member, is abutted on the surface of the intermediate transferring member so that a conditioning process for reducing the surface energy of the surface of the intermediate transferring member is provided. In this method, by adding a substance for reducing the surface energy to the intermediate transferring member, the surface energy of the intermediate transferring member is reduced.    (2) Linear velocities of the intermediate transferring member and the transferring member on which toner on the surface of the intermediate transferring member is transferred are made different from each other; thus, in this method, “the image loss phenomenon” at the time of the transferring process is reduced.    (3) A method in which zinc stearate is applied to the intermediate transferring member as a lubricant.
As described above, in the intermediate transferring member also, there have been demands for a high mold-releasing property and a sliding property, and various methods have been proposed to achieve these properties; however, these methods have not provided sufficient solutions that achieve desirable mold-releasing property and sliding property, and maintain the high mold-releasing property and sliding property for a long time.
Furthermore, in each of the members, such as the charging member which is abutted on the surface of the photoreceptor, or is placed close to the surface thereof so as to charge the surface of the photoreceptor, the transferring member for transferring the toner image of the surface of the photoreceptor to a transferring material, the transferring member for transferring the toner image of the surface of the intermediate transferring member to a transferring material and the conveying member for conveying the transferring material, the following problems are raised when the mold-releasing property is low on the surface thereof.
In the case where the surface mold-releasing property of the charging member which is abutted on the surface of the photoreceptor or is placed close thereto so as to charge the surface of the photoreceptor is low, residual toner and external additives and further paper powder and the like tend to adhere as time has elapsed and it is difficult to remove these; therefore, it becomes difficult to evenly maintain the charging state on the surface of the photoreceptor, resulting in a defective image due to an insufficient charging process.
In the case where the surface mold-releasing property of the transferring member for transferring the toner image of the surface of the photoreceptor to a transferring material and the transferring member for transferring the toner image of the surface of the intermediate transferring member to a transferring material is low, the toner and external additives and further paper powder and the like tend to adhere thereto as time has elapsed, and are hardly removed; therefore, it is not possible to form a sufficient transferring electric field, resulting in a defective image due to a defective transferring process. Further, this phenomenon is particularly conspicuous in the case of the transferring member for transferring the toner image of the surface of the intermediate transferring member to a transferring material.
In the case where the surface mold-releasing property of the conveying member for transporting a transferring material is low, toner powder, paper powder and the like floating inside the machine tend to adhere thereto as time has elapsed, and it is difficult to remove these; therefore, stains on the surface of the conveying member are always transferred onto the transferring material.
As described above, with respect to the electrophotographic members, in the case where the mold-releasing property and the sliding property on the surface are low, various problems are raised; therefore, there have been strong demands for a member which has high mold-releasing property and sliding property on its surface, and also maintains the mold-releasing property and sliding property for a long time.